An optical device for transmitting light, also known as a waveguide, comprises a light-guiding portion or “core” and a cladding portion for confining light in the core. The device may comprise a waveguide having a core that is circular (optical fibers) or rectangular, and surrounded in cladding of lower refractive index. The device may also be a plane waveguide, likewise comprising a guiding layer and cladding, and deposited on a substrate acting as a support. Waveguides based on silica glasses have been in existence for a long time. Waveguides based on organic polymers constitute a more recent development.
The ability to modify the refractive index of a waveguide finds numerous applications in optical components. Thus, wavelength-selective filters are essential components for communications systems. Filters enable spectral separation to be performed by reflecting at certain wavelengths and transmitting at others. The filter function is performed in particular by diffraction gratings known as “Bragg” gratings. A Bragg grating is a wavelength-selective reflector having a periodic structure that establishes index modulation. It is characterized by its passband, i.e. the range of wavelengths that it allows to pass. A given array generally presents a reflection maximum at a certain wavelength λb referred to as the “Bragg wavelength”. It is defined by the relationship: λb=2.neff.Λ where Λ is the pitch of the Bragg grating and where neff is the mean effective refractive index of the guided fundamental mode incident on the grating. When the index can be modified, the Bragg wavelength λb can also be modified, so the filter becomes tunable. Bragg gratings can be made by optical, mechanical, or chemical methods that lead to a physical modification of the support. It has recently become possible to transform a small portion of a waveguide into a filter by inducing periodic variation in the refractive index of the core in the form of lines that are regularly spaced apart at a fixed distance;
In general, changing the mean effective refractive index neff of a simple waveguide enables an optical wavelength in a system to be adjusted. By way of example, this can be used to achieve fine adjustment of the characteristic frequencies of the system. Tunable lasers, tunable filters, and modulators are other examples of components that make use of such variable-index waveguides.
By way of example, mention can be made of a two waveguide interferometer of the Mach-Zehnder or Michelson type, which is used to obtain, in particular, a tunable filter or a modulator. Under such circumstances, the refractive index in one of the two waveguides is caused to vary so as to modify the transmission of the interferometer.
A Fabry-Perrot filter is a multiple wave interferometer in which a change of index causes resonant frequencies to be shifted and which can be used to obtain a tunable filter. A tunable laser is another particular case of this type of filter where the optical length of the cavity can be adjusted so as to control the emission wavelength very precisely. To do this, a phase section is disposed within the cavity, which phase section is a waveguide of modifiable index.
Mention can also be made of arrayed waveguide multiplexers/demultiplexers sometimes referred to as “phasars” or by the initials “AWG” (arrayed waveguide grating). The ability to modify the refractive index of waveguides can be used for tuning the spectral response of the components.
Naturally, it is also possible to modify the index of a simple waveguide that does not form part of an interferometer system. For example, in a junction, it can be important to modify the refractive index of one waveguide relative to another. For example, in a Y junction, by changing the index of one of the two outlet arms, it is possible to direct the inlet signal to one outlet or the other. This constitutes a switch known as a digital optical switch. The need for such systems is very important in all switching applications.
European patent EP-0 496 278 describes an optical device whose refractive index is controlled by a method based on the principle that the refractive index of a material containing mobile ions, i.e. an ionic conductor, varies reversibly as a function of the electric field applied thereto. The optical device comprises an ionic conductor made of a transparent material of high molecular weight containing mobile ions, and at least one pair of electrodes facing each other across the ionic conductor and in contact with the ionic conductor. When an electric field is applied to the conductor by means of the electrodes, its refractive index is modified in at least one ionic conductor zone, depending on the applied electric field. Under the effect of the potential difference applies across the electrodes, ions contained in the conductor move through the material so that the refractive index of the ionic conductor increases at one of its interfaces with the electrodes and decreases at its other interface. The device operates at low voltage, e.g. 20 volts (V).
The capacitance of the electrodes described is very small since ions can accumulate only on their surfaces. As a result, as soon as the voltage is no longer maintained, the change in index at the interface disappears quickly by self-discharge. In addition, the change in refractive index as obtained in that way is restricted to the contact interface between the ionic conductor and the electrodes. As a result the change is difficult to reverse and leads to an aging phenomenon. Finally, that device is not easy to use. It has the drawback of requiring its electrodes to be permanently maintained at a voltage that is relatively high (20 V) in order to conserve the desired refractive index, thereby leading to non-negligible consumption of electricity.